This invention relates to tags and, in particular, to tags for use in article surveillance systems.
Article surveillance systems are known in the art wherein magnetic tags are affixed to articles and used to maintain the articles under surveillance. Humphrey, U.S. Pat. No. 4,660,025, issued Apr. 21, 1987, and the patents cited therein disclose magnetic article surveillance systems of this type.
In such magnetic surveillance systems, an alternating magnetic field is formed in a surveillance zone and a magnetic tag passing through the zone causes a perturbation to the field. This perturbation is detected and used to activate an alarm, indicating the presence of the tag and the article carrying the tag in the zone.
In these systems, the extent of the surveillance zone and the reliability of detection is constrained by the physical laws associated with magnetic fields. It is well known that a magnetic field decreases in magnitude at a cubic rate as a function of distance. Therefore, the distance over which a magnetic field can travel is limited.
To compensate for this decrease in field strength, magnetic surveillance systems have been required to use magnetic fields of relatively high strength within the surveillance zone. However, the need to use fields of high strength increases the equipment cost of the magnetic system.
Other attempts to compensate for the drop off in magnetic field as a function of distance have involved increasing the sensitivity of the system receiver. However, increasing the sensitivity of the system receiver, makes the system more prone to interference from background noise. Accordingly, the cubic drop off of the magnetic field continues to be a governing constraint in designing magnetic article surveillance systems.
Another constraint in magnetic systems is that, in certain instances, ordinary objects passing through the surveillance zone can result in false alarms. This effect can be minimized by decreasing the sensitivity of the system receiver to all perturbations, except those generated by valid tags. However, this often results in decreasing the desired range of the system and/or increasing the cost and complexity of the receiver.
It is therefore an object of the present invention to provide a tag for an article surveillance system which is less prone to result in the above disadvantages.
It is a further object of the present invention to provide an article surveillance system tag which is responsive to applied fields whose strength drops off with distance at a lesser rate than for magnetic fields.
It is further object of the present invention to provide an article surveillance system tag that responds to an electrostatic field.